From personal preference, I would rather buy a brand-new cheaper luggage rather than a used branded/ expensive type on a bargain.

Basically, you don't know the history of that bag.

Polycarbonate bike helmets are supposed to be replaced after a crash, even with no visible damage because of the question of its structural integrity. As a corollary, you are not sure how many times that bag has been slammed to the ground or aircraft hold. I'm pretty sure the owner may not know that much about it as well.

Most important of all, you do not know what the previous owner transported inside that bag/ luggage - anything from stinky food, uncovered dirty shoes, or something illegal.

Just my 2 cents.